In co-pending application Ser. No. 3,324 filed Jan. 15, 1979, in the name of Harold S. Stratton and assigned to the same assignee as the present application, there is described a propulsion system using liquid propellant in which propellant from a relatively lowpressure source is injected into the combustion chamber at a high pressure by a regenerative arrangement which utilizes the pressure in the combustion chamber to power the injection mechanism. The present invention is an improvement on the injector mechanism therein described which also utilizes the combustion pressure to provide the injection pressure. However, the present invention provides much more precise control of the quantity of propellant injected as well as more precise control of the time interval over which the fuel is injected, resulting in a more fuel-efficient operation of the propulsion system.